Copolymers of ethylene with one or more .alpha.-olefins and optionally a minor amount of polyenes are known which, after vulcanization, exhibit elastomeric type properties. The most common of these are copolymers of ethylene with propylene (EPR), which can contain small amounts of non-conjugated diene monomers (EPDM).
Thanks to their good resistance to high temperatures, chemical and atmospheric agents, and their dielectric properties, these elastomers find uses in various application fields. Principally they can be used as additives for lubricating oils, covering materials in building, electric insulators, components in the automotive industry or modifiers in mixtures with thermoplastic polymers.
Elastomeric copolymers of ethylene with .alpha.-olefins different from propylene, at the date of the present invention, do not have interesting industrial applications.
The above mentioned ethylene copolymers are generally prepared in the presence of Ziegler-Natta catalysts.
Natta et al, for example, in "La Chimica e l'Industria, 41, 769 (1959)", describe copolymerization reactions of ethylene with 1-butene in the presence of vanadium halides and trialkyl aluminium compounds.
The copolymers so obtained have a high tendency to produce blocks of .alpha.-olefin comonomers in the chain, and this gives rise to crystallinity negatively influencing the final characteristic of the product. Therefore, in order to obtain vulcanized copolymers which are endowed with good elastomeric properties, it is important that the distribution of the monomeric units within the chain is as homogenous as possible.
Zambelli et al., in "Makromol. Chem., 115, 73 (1968)" describe the preparation of copolymers of ethylene and 1-butene in the presence of a catalytic system comprising VCl.sub.4, AlEt.sub.2 Cl and anisole. The copolymers obtained have a tendency to an alternating distribution of the comonomer.
More recently, ethylene copolymers of this type have been prepared in the presence of homogeneous catalysts obtained from metallocenes and aluminoxane compounds.
U.S. Pat. No. 5,001,205, for example, describes a process for the preparation of elastomeric copolymers of ethylene with .alpha.-olefins, in which the product obtained from the reaction of methylaluminoxane (MAO) with a bis-cyclopentadiene compound of Zr, Ti, or Hf, such as bis(tetrahydroindenyl) zirconium dichloride, ethylene-bis(tetrahydroindenyl) zirconium dichloride, or dimethylsilandiyl-bis(tetrahydroindenyl)zirconium dichloride, is used as a catalyst. The working examples only regard ethylene/propylene copolymers.
The homogeneity of the distribution of the .alpha.-olefin monomeric units in the copolymers obtained operating in the presence of these types of catalyst, although turning out improved with respect to the copolymers obtained from Ziegler-Natta catalysts, is still not totally satisfactory.